nv50/ir: (trivial) initialize pointer to silence warning

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>

src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp

@@ -2141,7 +2141,7 @@ Converter::handleINTERP(Value *dst[4])
 {
    // Check whether the input is linear. All other attributes ignored.
    Instruction *insn;
-   Value *offset = NULL, *ptr = NULL, *w;
+   Value *offset = NULL, *ptr = NULL, *w = NULL;
    bool linear;
    operation op;
    int c, mode;